vrend: Emulate Multisample BGRA format on GLES
BGRA formats don't support texture storage on GLES, but GLES supports creating multisample textures only by using glTexStorageMultisample, so switch to a swizzled RGB format.
v2: Simplify handling of whether we should emulate (Gurchetan)
Fixes #133 (closed)
This was split out from !285 (merged) since these are actually two different bugs.